When anonymous voting is used it will only let a new vote be cast every 5 minutes per ip address otherwise you would have bots posting all day over and over skewing results, so if any vote was made whether authenticated or not from the same ip address then it will show the your vote has been counted message. If you wait long enough then it should allow another vote.
If the voter is authenticated it will only let him vote one time. I suppose if you log out then visit again from the same ip it is a minor bug if it says your vote has been counted, but it is doing that because you are at the same ip address as your previous vote I don't think an anoymous user from a different ip address would see that. I will look into it when I get a chance.
Best,
Joe